Thin film photovoltaic (PV) modules (also referred to as “solar panels” or “solar modules”) are gaining wide acceptance and interest in the industry, particularly modules based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy (sunlight) to electricity. For example, CdTe has an energy bandgap of 1.45 eV, which enables it to convert more energy from the solar spectrum as compared to lower bandgap (1.1 eV) semiconductor materials historically used in solar cell applications. Also, CdTe converts energy more efficiently in lower or diffuse light conditions as compared to the lower bandgap materials and, thus, has a longer effective conversion time over the course of a day or in low-light (e.g., cloudy) conditions as compared to other conventional materials.
Typically, CdTe PV modules include multiple film layers deposited on a glass substrate before deposition of the CdTe layer. For example, a transparent conductive oxide (TCO) layer is first deposited onto the surface of the glass substrate, and a resistive transparent buffer (RTB) layer is then applied on the TCO layer. The RTB layer may be a zinc-tin oxide (ZTO) layer and may be referred to as a “ZTO layer.” A cadmium sulfide (CdS) layer is applied on the RTB layer. These various layers may be applied in a conventional sputtering deposition process that involves ejecting material from a target (i.e., the material source), and depositing the ejected material onto the substrate to form the film.
Solar energy systems using CdTe PV modules are generally recognized as the most cost efficient of the commercially available systems in terms of cost per watt of power generated. However, the advantages of CdTe not withstanding, sustainable commercial exploitation and acceptance of solar power as a supplemental or primary source of industrial or residential power depends on the ability to produce efficient PV modules on a large scale and in a cost effective manner. The capital costs associated with production of PV modules, particularly the machinery and time needed for deposition of the multiple thin film layers discussed above, is a primary commercial consideration.
In particular, sputtering from a semiconducting target that is resistive in nature (e.g., cadmium sulfide) can be difficult to implement at a high rate with sufficient uniformity for mass production of PV devices, particularly when the target has a relatively large surface area.
Accordingly, there exists an ongoing need in the industry for an improved system for economically feasible and efficient large scale production of PV modules, particularly CdTe based modules.